Phase Change Material

My friend and colleague, Eric Bastow, got back last month from an IPC standards meeting with some interesting news for those of us who supply and use solder paste. Here I’m talking about everything from standard SMT printing and Power Semiconductor die-attach solder paste (type 3 and 4) down to PoP and waferbumping solder pastes (type 5, 6, 7 etc). I had heard that there were some changes in the way the powder types are categorized and wanted to learn more. Here is what we discussed:

[Andy Mackie] What is the current status of solder powder “type” designations from the new IPC J-STD-006B (Oct 2009)?

[Eric Bastow] In the original J-STD-006B (Oct 2008) and its two amendments, a solder particle size distribution (PSD) table, Table 3-1, was included as part of the standard to define the different powder “types”: 3, 4, 5 and so on. However, this table has been removed from the version published exactly 1 year later (October 2009) and also, somewhat confusingly, called J-STD-006B. This latter standard refers the reader to the old J-STD-005 (Solder Paste) for powder type determination by PSD, tables 2A and 2B.

[Andy Mackie] So how are solder powder types currently (May 2010) defined by the IPC? 

[Eric Bastow] The responsibility for defining the powder size distribution for the respective types now goes by default to IPC Task Group 5-24b, which maintains the J-STD-005 and its amendments and associated documents. This standard and its amendment were created in the early 90’s, and then published in January 1995, when even type 4 paste was uncommon at best, so its relevance now in the second decade of the third millennium is rather questionable, particularly given the enormous changes in solder powder manufacturing methodology and analytical characterization that have occurred in that timeframe.

[Andy Mackie] I understand that there are even some concerns about the test methods used to define the PSD.

[Eric Bastow] Yes, very much so. It is interesting to note that the original J-STD-006B Table 3-1 recognized that “Types 5, 6 and 7 are shown as general industry accepted size ranges for development purposes. Current listed methods for measuring these particle sizes may not be accurate enough for exact size and range distribution”. That initial sentence is very revealing about the tentative nature of these “type” definitions. These same concerns were raised at the J-STD-005 meeting at APEX in April 2010, and I also raised issues about the relevance of the test methods (see below) that were in use.

Note that none of these addresses the possibility of pure solder powder being the sample.

[Andy Mackie] How did you and Indium Corporation drive the Solder Paste Task Group (5-24b) into the next phase?

[Eric Bastow] We realized that using 15year old test methods and standards for solder powder based exclusively on extraction from solder paste would raise serious concerns with our customers. As a start, Indium Corporation suggested round-robin testing amongst the various solder powder suppliers. The testing will involve the use of the in-house measurement techniques of those suppliers on representative powder samples from each of those suppliers, to see what sort of data scatter is observed. We helped the task group to recognize that defining the particle size distribution of the various types, especially the finer types, does not make much sense without first determining a reliable and repeatable method of measuring the particle size.

Once that is complete, we can begin to define what we mean by each powder type, and also if there is a need for such “hybrid” categorizations as type 4.5.

[Andy Mackie] Eric: thank you, and please keep up the good work.

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The interesting thing is that it will not affect the way Indium Corporation supplies or manufactures solder powder and paste materials according to our customers’ needs: just how we define them.

Cheers!   Andy

下周在硅谷地区，Indium公司会参加Semi-Therm---“热管理”的盛会！

在微型化(miniaturization)的今天，我们的各种电子产品都越来越小，但是性能更多更齐全，消耗功率更大，那么单位面积上所要求的散热，也应该越来越多。 这是为什么这几年热管理(Thermal Management), 热管理材料(Thermal Interface Materials---TIM) 会成为我们讨论的热门话题。  举个简单的例子吧， LED灯，面临的最大挑战之一就是如何解决热管理问题。

传统的热管理材料， 有导热硅胶(Grease), graphite, 相变导热材料(phase-change materials)等。这些化学材料都很便宜，也能起到一定的导热性能。 但是因为物理性质的限制，金属的导热性能的普遍都比这些化学物质的导热性能强很多。 比如说化学材料，一般能做到几瓦每摄氏度的热传导(thermal transfer efficiency), 已经很不错了。但是金属材料，如纯铟片(pure Indium foil)，在一定的表面处理后，是86W/⁰C.  

好了，先写到这里。 下周再和你分享更多展会等热管理材料的信息！ 

PS:  除了在下周在Semi-Therm展会和客户等一系列活动，3月份我会踏上一段新的征程，转去西雅图工作，负责美国西北部一系列的销售/技术活动。期待与你分享更多新技术，工艺信息，行业动态，客户疑难等！ Cheers!  

Generically, a phase change material is one which will store or release energy when it changes phase from solid to liquid or liquid to solid. According to this generic classification, there are 4 general categories of phase change materials.

• Salt Hydrates such as Sodium Sulfate, Calcium Chloride, Sodium Acetates,etc
• Eutectic Salts
• Paraffins
• Non-Paraffin Organics

These phase change material categories are not all-encompassing, however. Other materials such as metals, eutectic or not, are used as phase change materials for their thermal energy storage and removal abilities. 

Nearly all soft solders classify as phase change materials according to their melting temperature. According to Maurice J. Marongiu from MJM Engineering, who conducted a webinar on phase change materials, the melting temperature of a typical phase change material is between 0-250ºC. Solders officially may melt at higher temperatures, such as the AuGe eutectic alloy which melts at 356ºC, however the majority of solders used melt below 250ºC. 

Phase change materials are a common occurrence in the world of thermal interfaces for electronics. Here, tighter commonalities between phase change materials can be found. For instance, the phase change temperature for these thermal interface materials is within the range of a common TIM junction temperature, which is typically lower than 100ºC. For this reason, when we consider a metal interface to be a phase change material in this industry, it is an alloy or material which changes phase below 100ºC.

When implementing a metal or non-metal phase change material into a thermal interface, there are some design considerations to be made:

• Phase change materials are applied as solid pads. At room temperature they are firm and available with specific dimensions which make them easy to handle. Consistent application should be inherent.

• Phase change materials each change phase at a unique temperature. The appropriate phase change material engineered for an application will have a phase change temperature reached within the normal operating cycle of the device.

• Phase change materials are designed to turn liquid in operation. The liquid phase of these materials will have a distinct viscosity. Depending on the material, clamping pressure and assembly orientation, the molten material may leak. Proper precautions should be taken to prevent material leakage, especially toward active electrical components if the phase change material is electrically conductive.          

• When reservoirs are created to contain a phase change material, these reservoirs must accommodate the liquid phase of the phase change material as well as the solid phase. As a phase change material changes from solid to liquid there is an increase in the material volume. If the phase change material expands and fractures the reservoir, this will lead to leaks and the eventual failure of the electronic device as the thermal interface becomes backfilled with air.  

Greenpeace has just updated their "Guide to Greener Electronics."  There are a couple of interesting tibdits that I took from their report:

1. They are really focusing on the phase out of BFR's and PVC from electronics.  They dropped HP and Dell down because they are loosening their timeline of BFR and PVC phase-out.  I find it interesting that they make no note of what the replacements should be.  This is concerning that the replacements could potentially be MORE toxic than what they are replacing.  It took years to fully characterize the situations where BFR's and PVC are of concern (dioxin formation and bioaccumulation).  In addition, all BFR's are not the same.  If companies are phasing out these materials, how can they do a full risk assessment of the replacements in one to two years?
2. They have added Antimony (Sb) to their list of materials that need to be phased out.  This can be challenging for a number of soldering applications.  Component manufacturers have been using Sn/Sb alloys inside their components.  Sn/Sb is the highest melting point Pb-Free alloy that actually solders reasonably well (other than Au/Sn which is 1000x as expensive).  The component guys are using this so that those alloys are not remelting when that component is assembled in a SAC SMT process.  Eliminating Sb will create a number of assembly challenges as well as potentially significant reliability issues.
3. By reading the summary of the report, they praise Apple for phasing out virtually all BFR's and PVC.  However, their ranking is still in the bottom half.  I will write more about this one in a future entry.

I am all for designing electronics for the environment, but I think there needs to be more focus on the consequences of making those design changes.  Are the alternatives actually any better?  What is the impact on product reliability and functionality?

A drop of liquid metal.

Gallium is corrosive to aluminum. This picture was taken after gallium reacted with a sheet of aluminum foil. Image source: http://sci-toys.com/scitoys/scitoys/thermo/liquid_metal/liquid_metal.html

Liquid thermal interface materials are available in two forms:

1. metals liquid at room temperature
2. phase change metals

The major difference between the two is in the temperature which these alloys become molten.

Liquid metals remain molten at room temperatures. The following three liquid metal alloys become liquid at temperatures below 30 °C.

– Indalloy 51 (62.5Ga, 21.5In, 16.0Sn)
– Indalloy 46L (61.0Ga, 25.0In, 13.0Sn, 1.0Zn)
– Pure Gallium

Phase Change Metals are applied in solid form and melt when exposed to heated junction temperatures. The most popular phase change alloy melts at 60°C.

– Indalloy 19 (51In, 32.5Bi, 16.5Sn)

The advantages to using these liquid metal thermal interface materials are many and include:

• Extremely Low Thermal Resistance
  • The resistance obtained with the Indalloy 51 was tested to be less than 0.015 cm2-°C/W.
  • Metal in its liquid form has virtually no contact resistance
• High Thermal Conductivity
  • As a completely metal thermal interface material, bulk thermal conductivity is premium
• Ability to Withstand dramatic Thermal Expansion Mismatch
  • The low flow stress of liquid metals allow them to maintain surface contact while substrates pump during power or temperature cycling
• Ultra Low Bondline Thicknesses
  • Liquid metals can be compressed to thicknesses below 0.001"

One difficulty encountered when using these alloys is the ability to contain them. All of the alloys which are liquid at room temperature contain gallium. Gallium is corrosive to various metals, especially when hot. As the temperature of the gallium is raised, it becomes increasingly corrosive, reacting through thicker layers in a short amount of time. One metal which gallium is very reactive with is aluminum. It will corrode through .002" thick aluminum foil within hours at room temperature, and at 500°- 1000°C, this reaction becomes much faster.

Gallium is non-reactive with other metals however such as molybdenum, tungsten, and nickel.

In addition to reacting with metals, gallium with also stick to non-metallic materials, making it difficult to package. Stay tuned for my next blog posting which will discuss packaging options for liquid metals.

The ultimate goal for a solar panel is to reach grid parity. In order to achieve this goal all the players in thesolar PV value chain have to do their part to lower the total system cost. The common view is that solar cell manufacturers play a critical role in achieving this goal. All the cost savings, design changes and performance improvements that a solar cell manufacturer can achieve has a multiplying effect across the value chain. This is a true statement, but the key enabling factors for all these improvements are advanced manufacturing and assembly materials.

One such example is a CIG Target for manufacturing of CIGS solar cells. Sputtering as we all know is a high throughput and high precision process. But this process requires the availability of high quality CIG target. A CIG target not only needs to be dimensionally accurate and highly dense but also needs to have homogeneous composition and grain sizes throughout. Moreover, adding to this complexity all the phases need to be tightly controlled inorder to obtain uniform sputtering. Innovations in hybrid consolidation processes have enabled high volume production of CIG sputtering targets bringing CIGS solarcells close to commercialization.

Metal Phase Change Materails Dispensed to mold a thermal interface gap, determining physical characteristics needed for a Thermal Interface Material

Looking for ways to troubleshoot or optimize your Thermal Interface materials?  One method which we have contrived to determine the performance of your thermal stack-up excluding the thermal interface material is to use a liquid metal.  This is a topic I have discussed at multiple conferences, but will discuss in-depth in a later posting.
 

Another method that can be used involves placing a phase change metal at the thermal interface to make a mold of the Tthermal interface material gap.  Some metal thermal interface materials melt at a low enough temperature to melt during chip operation or under mild heat.  At room temperature however, they re-solidify, taking a mold of the thermal interface material gap.  By disassembling your thermal stack-up with the phase-change metal, you can view any non-planarity of substrates or surface roughness through inspection of the metal thermal interface material.  It is unlikely that the interface material will completely squeeze-out or appear with a smooth mirror-like finish because no machined parts are perfect.  This method will demonstrate just how far off perfect your material design has come.    

Over the past few months I have received many inquiries from engineers who are just beginning to consider metal alloys as thermal interface materials. In looking at their existing stack-up, more and more are beginning to realize that the bottleneck in improving thermal performance of their device is through the interfaces between die and spreader or spreader and heat sink. The air gaps left behind from their pumped-out grease and high resistivity found in thermal tape, or best of all, thick thermal pads are not cutting it.

From the moment they realize the potential of indium-based materials, they are impressed by the numbers but the assumption is that pure indium foil is the only option because of its superior conductivity at 86W/(m-K). The options go far beyond this however, and I encourage considering all the options before making a decision.

Preforms:
Preforms are available in pure indium as well as alloys of indium such as 97In3Ag, 90In10Ag, or 52In48Sn. Other alloys such as those with a tin base are also considered for some devices. These preforms are available in many thicknesses from 0.001" and greater. Preforms may be applied and compressed to contact surfaces without heat application or reflowed to produce a soldered interface.

Phase Change Metals:
These materials are available as solid preforms and during elevated temperature operation, the solder melts conforming completely to all contacted surfaces. One of the most popular alloys for this use is Indium's alloy #19 composed of InBiSn. This alloy melts at 60°C.

Liquid Metals:
Metal alloys which have melting temperatures near or below room temperature can be dispensed onto substrates and if constrained to a specified area will continue to flow and remain in intimate contact with surfaces to provide impressive conductivity. Indium alloys #46L and 51 may be used in this way.

While all of these solutions may not be practical for every device, metal interfaces can be optimized to produce high performance interfaces which remove the thermal bottleneck from the interface location and provide long-term reliability. For assistance in finding the optimal material, please contact a member of Indium's technical staff.